868 



THE SENSE OF SIGHT 



yond this point (B) an inverted image will be obtained, and the light 

 in the pupil will appear to move against the rotation of the mirror, 

 whereas inside A the image (C) is erect, and the light seems to move 

 with the rotation. 



In myopia, the point of reversal lies close to the eye. Con- 

 sequently, if the observer finds that, on throwing light into the eye, 

 the light in the pupil is against the rotation, he must be beyond the 

 point of reversal. He should then approach the observed eye slowly 



Fio. 468. DIAGRAM TO ILLUSTRATE THE LOCATION or THE POINT OF REVERSAL AS OB- 

 TAINED BY THE SHADOW TEST. 



until he finds this movement to be with the rotation. These obser- 

 vations should be repeated until this point has been accurately local- 

 ized. The distance between this point and the eye should then be 

 measured with the ruler, because it represents the focal distance of 

 the lens necessary to correct the myopia. Thus, if it is possible 

 to obtain an erect movement at a point 55 cm. from the eye and a 

 reversed movement at 80 cm., the exact point of reversal will be at 

 67 cm. The myopia equals in this case 1.50 D. 



Fio. 469.- 



-DIAGRAM TO ILLUSTRATE THE LOCATION OF THE POINT OF REVERSAL AS OB- 

 TAINED BY THE SHADOW TEST IN THE HYPERMETROPIC EYE. 



In hypermetropia the rays are emitted divergently and hence, a 

 point of reversal cannot be present. The observer then finds that the 

 movement of the light remains with the rotation, no matter how far he 

 withdraws from the eye. A convex lens should now be interposed to 

 form a point of reversal at a convenient distance from it, thereby ren- 

 dering it artifically myopic (Fig. 469). This point of reversal having 

 been ascertained with the ruler, the degree of myopia represented by it 

 is then subtracted from the total strength of this lens. The remainder 



